The present invention relates to systems for converting mechanical energy to electrical energy; and, more specifically, to power conversion systems employing a relatively small engine.
In general, power conversion systems comprising a generator and an energy source, such as an engine, a motor, or a turbine, are well known. The generator typically comprises a rotor and a stator arranged for rotation relative to each other. Generally, the rotor is driven by the energy source, often mounted on the shaft of the engine, motor, or turbine. The rotor typically generates a magnetic field (using either permanent magnets or windings) that interacts with windings maintained on the stator. As the magnetic field intercepts the windings, an electrical current is generated. The induced current is typically applied to a bridge rectifier. The bridge rectifier output is sometimes regulated and provided as a DC output of the power conversion system. In some instances, the rectified signal is applied to an inverter to generate an AC output.
Portable power conversion systems find particular utility as: power sources for lights and small appliances used, for example, at construction or camping sites; power sources used in connection with recreational vehicles; and vehicular battery charger/jump start units. For vehicular units, it is desirable that the unit be capable of both charging a vehicle battery and jump starting a vehicle. When charging, the unit should deliver a tapered charge to the battery and avoid over-charging the battery. When jump starting, the unit should deliver full power. Vehicle electronics can be damaged by voltages that exceed a critical voltage, for example, 18 volts. Accordingly, precautions must be taken to ensure that the critical voltage is not exceeded.
It is also desirable that a power conversion system be able to accommodate wide and rapidly occurring variations in load, i.e., variations in output current. For example, when an incandescent lamp with a cold filament is xe2x80x9cplugged inxe2x80x9d to the generator, the generator is presented with extremely low resistance, resulting in an extremely high output current, often in excess of ten times the average output current. In the absence of special provisions, components typically must be rated for the anticipated peak output current rather than for the much lower magnitude of the average output current. The requirement for components rated for peak output current much higher than the average output current tends to add considerable expense to the generator.
It is particularly desirable that a power conversion system be lightweight and inexpensive. Generators which use permanent magnets to generate the requisite magnetic field tend to be lighter and smaller than traditional wound magnet generators. However, the power supplied by a permanent magnet generator has historically been difficult to regulate or control. The voltage supplied by such a generator varies significantly according to the speed of the rotor. In addition, the output voltage tends to vary inversely with the output current, i.e., as the output current increases for a given load, the output voltage drops.
Commonly assigned U.S. Pat. No. 5,886,504 describes, among other things, a lightweight, compact power conversion system wherein fuel economy, noise abatement, and the ability to accommodate widely and rapidly varying loads is facilitated through automated throttle control. The system includes an engine, with a throttle; an electromechanical throttle actuator (control device), a permanent magnet alternator; a controlled rectifier; an inverter; and a controller. The throttle control device controls the setting of the throttle in accordance with control signals from the controller. In one embodiment, rotor RPM is maintained at the lowest value necessary to provide a desired output voltage across the load. In another embodiment, the throttle is employed as a mechanism for controlling output voltage and to avoid over-current conditions so that damage to components caused by current surges due to variations in load are avoided by sensing an impending over-current condition, decreasing the output voltage by a predetermined amount or to a predetermined level, then gradually increasing the output voltage to bring the system back to a desired operating condition. A variety of suitable throttle control mechanisms are described.
In the event of a significant decrease in load, such as when a lamp is xe2x80x9cunpluggedxe2x80x9d from the generator and the throttle is already opened relatively wide, the engine speed will increase suddenly. A sudden increase in engine speed, if uncontrolled, can cause engine damage, may cause a rail voltage surge that can damage system electronics, and, if the generator is connected to a vehicle battery, can also cause damage to a vehicle""s electronics. An inexpensive mechanism is desirable for avoiding potentially damaging excessive engine speed and voltage surges.
It is also desirable that the system include an inexpensive but reliable mechanism for detecting short circuit loads connected across the system output terminals and for providing suitable indications of overload conditions to a user.
The noise generated by a small internal combustion engine operating at relatively high speed is significantly greater than the noise generated by the same engine at lower speed.
Accordingly, to reduce noise, it is desirable to operate the engine at relatively low speed to the greatest extent possible. However, the torque generated by the engine at low speed can be considerably less than the torque generated by the engine at higher speed. At relatively low speed the engine may not be able to generate sufficient torque to start supplying power when the output terminals are connected to loads having a relatively high initial power requirement.
A power conversion system, in an implementation according to various aspects of the present invention includes an engine coupled to drive an alternator, a rectifier circuit, and an ignition controller. The alternator includes a stator and a rotor that cooperate when driven to provide a current supplied to the rectifier circuit. The rectifier circuit provides output power having a voltage. The ignition controller supplies energy to the spark plug of the engine. If the voltage exceeds a predetermined limit, the ignition controller supplies less energy to the spark plug to decrease engine drive.
In another implementation, according to various aspects of the present invention, the stator includes a plurality of windings (or a tapped winding) and the system further includes an engine throttle controller and a winding control circuit. The winding control circuit selectively couples one or more windings of the plurality (or one or more turns of a tapped winding) to the rectifier circuit. The winding control circuit cooperates with a throttle controller to facilitate provision of a predetermined output power with a relatively low engine RPM for noise abatement.